Fecal occult blood (“FOB”) is a good indicator for monitoring bleeding from the gastrointestinal (GI) system. Since it may take several years for some colon polyps to transform into colorectal cancers, the detection of bleeding colon polyps is an effective way to screen for colorectal cancer at an early stage. The implementation of a procedure to screen for occult blood in fecal samples for adults of 50 years or older has reduced the incidences of colorectal cancer by 20% and mortality by 30%.
Currently available FOB tests include a chemical test that detects hemoglobin metabolites in fecal samples. The test employs an oxidizable substrate (such as guaiac) that produces a colored product in the presence of peroxide and hemoglobin. Since most animal heme molecules, plant ingredients and common vitamins can also catalyze the hydrogen peroxide reaction, diet control is critical to specificity of this type of chemical test. A strict diet control poses a serious patient compliance problem. On average, a compliance rate of lower than 10% is not unusual. Further, chemical tests generally have a limited sensitivity. For example, a FOB test using guaiac as substrate has a test sensitivity of 50 μg/ml, with an improved version having a test sensitivity of 20 μg/ml.
A sandwich immunoassay for hemoglobin has been developed, which provides improved test sensitivity and specificity as compared to chemical tests. The need for diet control is also eliminated for such immunoassay.
Hemoglobin is a labile protein; it degrades rapidly in human GI system. Hemoglobin is also susceptible to degradation during shipping and storage after fecal samples are collected. All these sample degradation problems affect the test accuracy of hemoglobin assays.
Transferrin is another biomarker for detecting fecal occult blood. In normal human blood the concentrations of transferrin and hemoglobin are 3 mg/ml and 150 mg/ml, respectively. On the other hand, the hemoglobin versus tranferrin ratio in fecal samples is approximately 5, which is much lower than the ratio (approximately 50) in blood. This observation indicates that transferrin is about 10 times more stable than hemoglobin in the GI tract. It has been reported that certain diseases may affect the levels of transferrin in blood and fecal samples to the extent that an FOB test based on transferrin alone may produce false negative results. A combination test for detecting both hemoglobin and transferrin may improve test sensitivity without sacrificing test specificity.
Another challenge for developing an FOB assay is that very few automated analyzers currently on the market can handle fecal samples. Therefore, a rapid onsite FOB test is desirable and in demand.